A known turbocharger system for an engine comprises a two-stage turbocharger that comprises high- and low-pressure turbines in series flow relationship and a bypass valve that is in parallel flow relationship to the high-pressure turbine and under the control of the engine control system. The engine control system processes various data to control the bypass valve such that exhaust back-pressure and engine boost are regulated in an appropriate way according to the manner in which the engine is being operated. The high-pressure stage can be designed to have a relatively smaller size that is optimized for low-end engine performance while the low-pressure stage can be designed with a relatively larger size for high-end performance.
Exhaust gas recirculation (EGR) is typically used over a wide range of engine operating conditions under various ambient conditions to aid in controlling tailpipe emissions for achieving compliance with applicable laws and regulations. Engine engineers may however deem it appropriate, without violating applicable laws and/or regulations, to temporarily suspend the use of EGR during certain conditions that affect engine operation in certain ways. Reasons for such temporary non-use of EGR may include the following: EGR is simply unnecessary; EGR has no significant effect; or EGR is actually detrimental. Another possible reason is that sufficiently precise control of EGR cannot be realized while such conditions prevail.
A turbocharger control strategy may take into account many engine operating parameters, including EGR, over a wide range of engine operation. When an engine operates while prevailing conditions require EGR to be inactive, a turbocharger control strategy may require adaptation for inactive EGR.
Failure to adapt the strategy for inactive EGR can cause the engine to operate in ways that are detrimental to engine performance and/or durability. One consequence is an undesired increase in exhaust back-pressure (EBP) caused by a turbocharger control strategy striving to increase boost when it is inappropriate to do so because certain prevailing conditions, such as an engine being cold or being extremely hot to the point of overheating stipulate that EGR be rendered inactive during those conditions.